1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having MOS transistor circuits.
2. Description of the Prior Art
Conventionally MOS transistor circuits driven by a power supply voltage of 3.0-5.0 V and having a threshold voltage of about 0.7 V have been normally used in a semiconductor circuit. Recently, however, requirements for high-speed performance and low power consumption have made it necessary to modify the conventional power supply voltage and threshold voltage. For example, it is essential that a semiconductor integrated circuit, for use in a portable appliance having its power supplied by a dry cell, be capable of operating with a low power consumption. The most effective approach to satisfy this need is to lower the power supply voltage. It is to be noted, however, that a transistor circuit operated with a lower power supply voltage and having its threshold voltage unchanged exhibits a slower performance. Hence, it is necessary to lower the threshold voltage when the power supply voltage is lowered.
It is known that the threshold voltage of a transistor is removed from a target level due to a variation in the fabrication process. For example, assuming that the target threshold voltage is 0.7 V, the threshold voltage of transistors in an actually produced circuit may vary between 0.6-0.8 V. Assuming that the target threshold voltage is lowered to 0.1 V in correspondence with a lowered power supply voltage, the threshold voltage of transistors in a circuit produced in the same process may vary between 0.0-0.2 V.
When the threshold voltage of a transistor is 0.0 V, a relatively large leakage current occurs so that the power consumption of the whole circuit becomes large. When the threshold voltage of a transistor is 0.2 V, a relatively small current flows when the transistor conducts, resulting in a slow operation speed of the circuit. While a variance of 0.1 V may not present a serious problem when the target threshold voltage is 0.7 V, the same degree of variance presents a serious problem when the target threshold voltage is 0.1 V.
One approach to correct a variation in the threshold voltage is to detect a well potential of a circuit and control a charge pump circuit connected to the well (for example, see "Tackling the parasite effect and maintaining the high performance trend", Nikkei Microdevices, July 1995).
It is also to be noted that, even when the threshold voltage is set to the target level, there is a need to reduce a leakage current only when the circuit is not in operation (that is, in a stand-by mode) for advanced lower-power consumption operation. One approach to reduce a leakage current when the circuit is in a stand-by mode is to switch between two types of external power supplies connected to the well potential of the circuit (for example, see "Tackling the parasite effect and maintaining the high performance trend", Nikkei Microdevices, July 1995).
While the above-described two approaches are effective to correct a variation in the threshold voltage and to reduce a leakage current, respectively, these two approaches can not be implemented at the same time because they are designed to control the well potential of a transistor using different approaches.
Conceivably, the leakage current can be reduced in a stand-by mode by providing transistors having different threshold voltages in the same circuit and disconnecting the transistors having a lower threshold voltage from a power supply using the transistors having a higher threshold voltage. However, when this approach is combined with the aforementioned circuit for correcting a variation in the threshold voltage, the circuit scale (i.e., size) becomes large and an additional step must be introduced in the fabrication process.